1. Field of the Invention
The present invention relates to a communication device and a communication method for continuously receiving multiple data (data packets) from a transmitter via a radio link, and relates to a radio communication terminal and a radio base station each including the communication device.
2. Description of the Related Art
Heretofore, in radio communications, a transmission power control technique has been used to control the transmission powers of a radio communication terminal and a radio base station. Precisely, in a downlink, a phenomenon called fading occurs in which the received power of a radio communication terminal suddenly fluctuates due to a movement of the radio communication terminal or reflection of radio waves by buildings or the like. Since such fading must be avoided, the transmission power in the downlink cannot be decreased without limitation. On the other hand, when the transmission power from a radio base station is too high, a transmission signal of a certain radio communication terminal interferes with a transmission signal of another radio communication terminal.
Accordingly, the communication quality in downlink must be maintained by controlling the transmission power of the radio base station in consideration of a fading influence, while avoiding fading and the like. For this purpose, conventional radio communication terminals employ a method of setting the reference quality for a service during communications, and of controlling the transmission power of a radio base station at a high rate so that communication quality in the downlink can follow the reference quality.
To be more precise, closed loop transmission power control composed of outer loop control and inner loop control is used. In the outer loop control, a target SIR (signal-to-interference power ratio) is controlled such that the communication quality such as the block error rate (BLER) or the bit error rate (BER) can become a target value (reference quality).
On the other hand, the inner loop control is made such that the SIR of a communication channel received by the radio communication terminal can be the target SIR. The radio communication terminal transmits a transmit power control (TPC) command to the radio base station according to a comparison result between the SIR of the communication channel and the target SIR. Here, the TPC command is information for controlling the transmission power.
In addition to the aforementioned transmission power control, retransmission control is used as a technique for securing the communication quality of radio communications performed between a radio base station and a radio communication terminal.
In the radio communications, data pieces are intermittently transmitted as divided data packets. For this reason, in the downlink retransmission control, the radio base station adds sequence numbers to data pieces. Then, by use of the sequence numbers, the radio communication terminal makes a notification as an acknowledgement (ACK) to the radio base station, the notification indicating that the data pieces have been successfully received. The radio base station retransmits a data piece that the radio communication terminal has failed to receive. As a result, highly reliable radio communications are established.
As a method for performing more highly reliable radio communications, another method has been proposed by combining the transmission power control and the retransmission control. In this method, in order to reduce a probability that a radio communication terminal will fail to receive an unacknowledge mode message (for example, a RRC CONNECTION RELEASE message indicating a completion of communications, or the like), the foregoing target SIR is increased at a predetermined timing before the radio communication terminal receives the unacknowledge mode message (see Japanese Patent Application Laid-open Publication No. 2004-112097).
By increasing the target SIR, the transmission power of the radio base station can be increased. Accordingly, the BLER is kept low for a certain period after the increase of the target SIR, and thereby the radio communication terminal more surely receives the unacknowledge mode message. In addition, in JPA 2004-112097, the transmission power of the radio base station is not increased for acknowledge mode data transmission, since a data piece is retransmitted through the aforementioned retransmission control.
However, in the following cases (1) and (2), the foregoing transmission power control does not function properly. As a result, even when the radio base station retransmits a data packet (hereinafter, called a PDU as needed), the retransmitted PDU is sometimes lost in a radio communication section, and this does not allow a receiver to receive the retransmitted PDU.
(1) A case where the SIR measurement needed for the outer loop transmission power control cannot be performed sufficiently due to a condition that data transmission is for AMR data of a silent period, is for PKTs not containing any data, or is of a bearer (SDCCH, an AMR and the like) using blind detection.
(2) A case where the reception quality is low due to an instantaneous change in a communication environment.
In these cases, since the radio base station performs retransmission repeatedly until the radio communication terminal transmits an acknowledgement (ACK) to the radio base station. As a result, the PDU is retransmitted so excessively that a problem arises that the buffer capacity and radio resources are wastefully consumed.
Moreover, when the radio communication terminal cannot receive a PDU even by use of the retransmission control, the following program occurs. In other words, as shown in FIG. 1, when the radio communication terminal cannot receive a PDU containing a transport channel reconfiguration transmitted from the radio base station, the radio base station and the radio communication terminal cannot switch to another channel at an activation time. As a result, the radio base station and the radio communication terminal get out of synchronization, and thereby are disabled to continue communicating with each other. Note that the activation time means a channel switch timing predetermined between the radio base station and the radio communication terminal before a channel switching is actually performed.